Targeted drug delivery (Muller, R; Keck, C (2004) J. Biotech. 113, 151) whose objective is to prolong, localize, target and have a protected drug interaction with the diseased tissue has been extensively studied during the past three decades. There are different types of drug delivery vehicles, such as, antibodies, proteins, vitamins, peptides, polymeric micelles, liposomes, lipoprotein-based drug carriers, nano-particle drug carriers, dendrimers etc. An ideal drug delivery vehicle must be non-toxic, biocompatible, non-immunogenic, and biodegradable (Scott, R; Crabbe, D; et al (2008) Expert Opin. Drug Deli. 5, 459) and avoid recognition by the host's defense mechanisms (Saltzman, W.; Torchilin, V. (2008). “Drug delivery systems” Access Science. McGraw-Hill Co.). The link between the delivery vehicles, in particular, antibodies and the cell-killing agent plays a critical role in the development of targeted drug delivery systems, as the nature of the linker significantly affects the potency, selectivity and the pharmacokinetics of the resulting conjugates (Zhao, R.; Wilhelm, S. et al, (2011) J. Med. Chem. 36, 5404; Doronina, S.; Mendelsohn, B.; et al, (2006) Bioconjug Chem, 17, 114; Hamann, P.; Hinman, L; et al. (2005) Bioconjug Chem. 16, 346). Four types of linkers had been used for preparation of cell binding agent-drug conjugates that have entered the clinic: (a) acid-labile linkers, exploiting the acidic endosomal and lysosomal intracellular microenvironment; (b) linkers cleavable by lysosomal proteases; (c) chemically stable thioether linkers that release a lysyl adduct after proteolytic degradation of the antibody inside the cell; and (d) disulfide-containing linkers, which are cleaved upon exposure to an intracellular thiol ((Zhao, R.; Wilhelm, S. et al, (2011) J. Med. Chem. 36, 5404).
Conjugates of cell-binding agents with drugs or modified chemical compounds via different types of linkers have been described (U.S. Pat. Nos. 4,680,338, 5,122,368, 5,141,648, 5,208,020, 5,416,064; 5,475,092, 5,543,390, 5,563,250 5,585,499, 5,880,270, 6,214,345, 6,436,931, 6,372,738, 6,340,701, 6,989,452, 7,129,261, 7,375,078, 7,498,302, 7,507,420, 7,691,962, 7,910,594, 7,968,586, 7,989,434, 7,994,135, 7,999,083, 8,153,768, 8,236,319, Zhao, R.; et al, (2011) J. Med. Chem. 36, 5404; Doronina, S.; et al, (2006) Bioconjug Chem, 17, 114; Hamann, P.; et al. (2005) Bioconjug Chem. 16, 346). Typically, in these conjugates, the cell-binding agents are first modified with a bifunctional agent such as SPDP (N-succinimidyl 3-(2-pyridyldithio) propionate), or SMCC (succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate); or SPDB (N-succinimidyl 4-(2-pyridyldithio)butanoate); to introduce an active disulfide or a maleimido moiety. Reaction with a thiol-containing cytotoxic drug provides a conjugate in which the cell-binding agent, such as a monoclonal antibody, and drug are linked via disulfide bonds or thioether bonds.
However, the use of the cell binding molecule-drug conjugates, such as antibody-drug conjugates (ADCs), in developing therapies for a wide variety of cancers has been limited both by the availability of specific targeting agents (carriers) as well as the conjugation methodologies which result in the formation of protein aggregates when the amount of the drugs that are conjugated to the carrier (i.e., the drug loading) is increased. Normally the tendency for cytotoxic drug conjugates to aggregate is especially problematic when the conjugation reactions are performed with the hydrophobic linkers. Since higher drug loading increases the inherent potency of the conjugate, it is desirable to have as much drug loaded on the carrier as is consistent with retaining the affinity of the carrier protein. The presence of aggregated protein, which may be nonspecifically toxic and immunogenic, and therefore must be removed for therapeutic applications, makes the scale-up process for the production of these conjugates more difficult and decreases the yield of the products.
Consequently, there is a critical need to improve methods for conjugating drugs/cytotoxic drugs to carriers (cell binding molecules) that minimize the amount of aggregation and thereby allow for as high a drug loading as possible through the application of a charged crosslinker.